1. Field of the Invention
The present invention relates to a composition and method for removing water, oxygen, and other oxidant and Lewis acid impurities from a flowing gas stream. The purification method does not contaminate the gas stream with added hydrocarbon impurities.
2. Description of the Related Art
The provision of high purity gas streams is critically important in a wide variety of industrial and research applications. The rapid expansion of vapor-phase processing techniques, e.g., chemical vapor deposition, in the semiconductor industry has been associated with the deployment and use of manufacturing equipment that is totally reliant on the delivery of ultra-high purity process gases at the point of use in the semiconductor manufacturing facility. Currently, over 5 billion dollars worth of such equipment is in use.
Considering the impurities which are present in gas streams involved in semiconductor manufacturing, it is to be noted that the growth of high quality thin film electronic and opto-electronic cells by chemical vapor deposition or other vapor-based techniques is inhibited by a variety of low-level process impurities. These impurities can cause defects that reduce yields by increasing the number of rejects, which can be very expensive. These impurities may be particulate or chemical contaminants. Particulates are typically filtered out of the gas stream using extremely efficient commercially available particle filters, with particle filtration generally being employed at the point of use.
Chemical impurities may originate in the production of the source gas itself, as well as in its subsequent packaging, shipment, storage, and handling. Although source gas manufacturers typically provide analyses of source gas materials delivered to the semiconductor manufacturing facility, the purity of such gases may change because of leakage into or outgassing of the containers, e.g., gas cylinders, in which the gases are packaged. Impurity contamination may also result from improper gas cylinder changes, leaks into downstream processing equipment, or outgassing of such downstream equipment.
Chemical impurities that are of special concern in semiconductor manufacturing processes include water, oxygen, and other oxidant and Lewis acid species such as aluminum, boron or zinc-containing species. In general, the key chemical impurities must be held at levels of a few parts per billion or lower.
In support of the requirement for high purity process gases, a number of types of gas purifiers have been introduced that remove chemical contaminants from the semiconductor process gases at the point of use. These gas purifiers employ a variety of sorption processes to remove impurities, including physisorption processes, e.g. gas adsorption by zeolites or activated carbon, or various chemisorption processes, where the impurities adsorb to and chemically react with a component or components of the purifier.
Particularly useful in-line purifiers are based on passive sorption processes, wherein the impurity species are adsorbed and chemically reacted with scavengers bound to or incorporated in porous inert support materials. Such purifiers are described in U.S. Pat. Nos. 4,603,148, 4,604,270, 4,659,552, 4,800,189. Because of their usefulness in purifying semiconductor process gas streams, where the requirements for purity are stringent, such purifiers have been the subject of much research and development activity, as well as significant commercial success. U.S. Pat. Nos. 4,761,395 (composition for purification of arsine, phosphine, ammonia and inert gases); 4,853,148 (hydrogen halide purification); 4,797,227 (hydrogen selenide purification); 4,781,900 (method of purifying arsine, phosphine, ammonia and inert gases); 4,950,419 (inert gas purification); 4,685,822 (hydrogen selenide purification); 4,925,646 (hydrogen halide purification method); 4,983,363 (apparatus for purifying arsine, phosphine, ammonia and inert gases); and 5,015,411 (inert gas purification method) describe this type of purifier and their disclosures are hereby incorporated herein. This class of purifiers is quite versatile, since the immobilized scavenger species may be varied and tailored to react with a large number of different impurities. Because the support material is usually porous, contact of the scavenger with the gas stream is extensive. Such gas purifiers are used to remove Lewis acid and oxidant impurity species, particularly water and oxygen, which have deleterious effects on the semiconductor manufacturing process. By varying the chemical identity of the scavenger, they may also be used to remove undesirable dopant species from the gas stream. Such gas purifiers can be very simple in design and operation, since purification occurs passively, simply by contact of the process gas stream with the scavenger.
As described in applicant's copending U.S. patent application Ser. No. 07/898,840, the disclosure of which is hereby incorporated herein, these purifiers can also be used advantageously in the back-diffusion scrubber mode. The purifier is outfitted with one or more endpoint detectors, and is positioned to purify the process gas stream before its entrance into the process tool, and also serves as an impurity scrubber that protects the gas supply against contamination caused by diffusion of one or more foreign components back into the supply lines. Back-diffusion can occur when mechanical components such as check valves and shut-off valves fail. Additionally, in low flow conditions, impurities can successfully diffuse against the convective forward flow. An example of a situation where back-diffusion is of concern is the case where an inert gas such as nitrogen is used to pressurize vessels containing liquids used in semiconductor manufacturing processes. Such liquids include sulfuric acid, isopropanol, acetone and the like, which can cause corrosion and contamination of the nitrogen supply system by back-diffusion under low flow conditions.
When the purifier is used for a back-diffusion scrubber, endpoint detection is critical. Back-diffusion is not planned for, and therefore it is impossible to predictively calculate the purifier's lifetime on the basis of flowrates, expected impurity concentrations, and so forth. Endpoint detection allows the immediate detection of a serious back-diffusion event, and the appropriate precautions to protect the gas supply may be mobilized. Use of two endpoint detectors disposed at separate points in the gas purifier's scavenger bed allows back-diffusion to be distinguished from normal exhaustion of the purifier. If the downstream endpoint detector signals purifier depletion before the upstream one does, back-diffusion can be diagnosed in a straightforward and simple way.
While the gas purifiers of the types described above are very effective at removing impurities from the process gas streams to very low levels, the scavengers may contribute low levels of hydrocarbon impurity to the gas streams being purified. U.S. Pat. Nos. 4,604,270 and 4,603,148 to G. M. Tom disclose scavengers in which alkyl metal compounds are immobilized by coupling them to an organic polymeric support, followed by pyrolysis to yield a dispersed phase of the metal hydride in the organic polymer matrix. For example, dried, porous styrene-divinylbenzene copolymer (PSDVB) beads are mixed with butyllithium and heated for a prolonged period in an oven to immobilize butyllithium species on the resin and largely convert the butyllithium to lithium hydride which is immobilized in the porous polymer beads. Such scavengers can contribute the butane elimination reaction by-product to gas streams being purified. Other purifiers that have alkylmetal-based scavengers or scavengers prepared from alkylmetal starting materials may manifest this same behavior.
Hydrocarbon impurities, even at very low levels, are highly undesirable in semiconductor process gas streams. In chemical vapor deposition processes, the high temperatures or plasmas in the reactor can cause decomposition of the hydrocarbon impurity and incorporation of carbon in the growing film. Carbon, a Group IVA element, is a dopant in compound semiconductors of the III-VI type.
Process gas purifiers based on other sorption principles such as metal eutectic alloy getters are sometimes employed, and these purifiers avoid the hydrocarbon impurity problem. For example, European Patent Application EP 470,936 describes removal of impurities from hydride gases by passing the hydride gas over a hydrogenated getter metal in a chamber. In particular, disiloxane may be removed from silane using hydrogenated Zr--V--Fe getter alloy. Gases which may be purified in this fashion include SiH.sub.4, GeH.sub.4, NH.sub.3, AsH.sub.3, SbH.sub.3 and PH.sub.3, all of which are used in semiconductor manufacturing. European Patent Application EP 365,490 describes a method for removing impurity gases from inert gases such as argon or nitrogen using a first sorbent of either a non-evaporable getter alloy of Zr--V--Fe or Zr--Fe and a second sorbent of a non-evaporable getter alloy of 5-50% Al, balance Zr. Both sorbents are pellets formed from alloy powder of average particle size below 125 microns, with the first sorbent being located at the gas inlet and the second at the gas outlet.
These metal eutectic alloy getters, while avoiding the problem of hydrocarbon contribution to the process gas stream, are not the simple, elegant systems that the passive sorption-based purifiers described earlier are. The getters must be operated at high temperatures, in the range of 300.degree. C. to 500.degree. C. At lower temperatures, e.g. about 25.degree. C., the scavenging capacities of the getters are low. Because the gases being purified can be highly flammable, e.g., silane or hydrogen, and because of the added complexity and expense involved in their use, the metal eutectic alloy getters are not the most desirable solution to the problem. Impurity reduction using the getters has been shown to be less efficient than competing sorption-based purifier technology.
The presence of even small concentrations of impurity species in the process gas streams employed in semiconductor manufacturing is potentially deleterious. Even small levels of impurities on the order of parts per million (ppm) can cause inconsistent electrical properties in semiconductor devices manufactured by deposition techniques using impurity-containing gas streams.
It therefore is an object of the present invention to provide a simple, rapid, and versatile purification system able to provide a high level of purification efficiency, with regard to removal of water, oxygen, and other oxidant and Lewis acid impurities, such as is required to protect semiconductor manufacturing processes.
It is a further object of the present invention to provide an improved scavenger characterized by high scavenging capacity with regard to removal of water, oxygen, and other oxidant and Lewis acid impurities, and that avoids previous problems of hydrocarbon contamination.
It is still another object of the invention to provide a method of making the aforementioned scavengers, and a process and apparatus for using the same to purify process gas streams, to remove water, oxygen, and other oxidant and Lewis acid impurities therefrom. Such scavengers and purifier systems can also be used in a back-diffusion scrubber mode to protect the integrity of the manufacturing process or gas supply system.